Refractory Plasmonics without Refractory Materials - Nano Letters

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Refractory Plasmonics without Refractory Materials Gelon Albrecht, Mario Hentschel, Stefan Kaiser, and Harald Giessen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03303 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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Nano Letters

Refractory Plasmonics without Refractory Materials Gelon Albrecht,

∗,†,‡

Mario Hentschel,

‡

Stefan Kaiser,

‡, †

and Harald Giessen

‡

†Max Planck Institute for Solid State Research, Heisenbergstraÿe 1, 70569 Stuttgart,

Germany ‡4th Physics Institute and Research Center SCoPE, University of Stuttgart,

Pfaenwaldring 57, 70569 Stuttgart,Germany E-mail: [email protected]

Abstract Refractory plasmonics deals with metallic nanostructures that can withstand high temperatures and intense laser pulses. Common belief was that refractory materials such as TiN are necessary for this purpose. Here we show that refractory plasmonics is possible without refractory materials. We demonstrate that gold nanostructures which are overcoated with 4 nm and 40 nm Al2 O3 (alumina) by an atomic layer deposition process or by thick IC1-200 resist can withstand temperatures of over 800 ◦ C at ambient atmospheric conditions.

Furthermore, the alumina-coated structures can withstand

intense laser radiation of over 10 GW/cm2 at ambient conditions without damage. Thus, it is possible to combine the excellent linear and nonlinear plasmonic properties of gold with material properties that were believed to be only possible with the lossier and less nonlinear refractory materials.

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Keywords plasmonic, refractory, thermal stability, gold nanostructures, THG, ambient atmosphere Plasmons, the collective oscillations of free electrons in a plasma, have become a widely used phenomenon in current optical applications, for example in solar cells, 1 optical biomaterial sensing, 2 and for nonlinear optics. 3 However, considering the vast range of applications, the utilized materials are still quite limited. Typically, either gold or silver are used, mainly due to their superior optical properties in the visible and infrared. Additionally, both materials exhibit a high optical nonlinear refractive index, rendering them excellent materials for nonlinear optics. Gold is furthermore chemically stable at ambient atmosphere, whereas silver quickly oxidizes. 4,5 Regardless of their chemical stability, gold, as well as silver, are prone to thermally induced deformation. Already at an ambient temperature of 100 ◦ C chemically grown gold nanoparticles start to deform, 69 minimizing their surface energy. 10 Within hours, the initially rod-like structures change into round structures. Intense laser exposure leads to heating and deformation of the structure. 1113 The structures become round during the heating process. For high uences in the mJ/cm 2 range, structures fabricated on a substrate are immediately detached from the substrate and scattered throughout the sample. 14 The deformation has direct impact on the plasmonic properties and will be visible in the optical spectrum of the nanoparticles. Most applications use the plasmonic resonance to absorb light or to detect changes in the refractive index around the plasmonic structure. All these applications rely on stable optical properties of the plasmonic structure. Therefore, bare gold and silver are not suited for applications involving elevated temperatures or intense illumination. To overcome this poor thermal stability, other materials were proposed to replace gold, namely the so-called refractory materials. A refractory material is dened by having a melting point beyond 2000 ◦ C. For the near infrared wavelength regime oxide materials such as indium-tin-oxide (ITO) were demonstrated. 15,16 Also nanostructured tungsten and molybdenum can be used in the infrared and withstand temperatures at least up to 900 ◦ C in 2


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vacuum. 17,18 Other materials with plasmonic properties in the infrared, however not refractory, are heavily doped GaAs and undoped InSb. 16,19 Especially the transition metal nitrides such as titanium nitride (TiN) and zirconium nitride have been studied in great detail, exhibiting plasmon resonances even in the visible and near-infrared wavelength range. 7,16,2028 However, despite their excellent thermal properties, basically all known refractory materials exhibiting plasmonic properties are prone to oxidization and are chemically unstable, in particular when compared to gold. For example, titanium nitride has a melting point of 2977 ◦ C, 29 yet it already oxidizes rapidly at 800 ◦ C and loses its plasmonic properties. 22,30 Consequently, these materials have to be kept under vacuum conditions 23 to prevent oxidization. Alternatively, the nanostructures can be buried in a protection layer, 17,31 which was shown to work for some materials and applications. Many commonly used dielectrics, however, are oxides. It was shown, that oxygen from the oxide can diuse into certain refractory materials, for example TiN, and cause oxidization. 17,22 Here, we demonstrate gold-based material systems for visible and near infrared plasmonic applications to fabricate thermally and chemically stable nanostructures, without the limitations of, e.g., TiN or bare gold. To realize such systems, we applied the concept of a protection layer to gold nanostructures. Our protection layer is not supposed to prohibit chemical reactions, but rather to prevent the deformation of gold. We described in a previous publication 32 that gold in a polymer matrix is more stable upon laser exposure. The general idea of this concept has been widely used for many years. 3336 While these publications report rst strong indications of the benets of cover layers, no systematic study of the temperature nor of the thickness dependence have been performed. Both quantities are of fundamental as well as of practical importance. For example, thickly covered structures are not useful for plasmonic sensing type experiments. Thus we signicantly extent the previously reported results by studying the temperature evolution from room temperature up to 1100 ◦ C, well above the gold melting temperature. We also show that already thin cover layers signicantly stabilize the structures. What is more, we demonstrate the superior

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performance of the covered structures upon intense laser illumination, also highly important for various applications. Besides the cover material, also the substrate material inuences the stability of the nanostructures. 37,38 Therefore, we compared suprasil and Al 2 O3 as substrate materials. As cover material, we choose Al 2 O3 and IC1-200, a polysiloxane-based spin-on dielectric material, used for multilayer lithography. Al 2 O3 is a refractory material, however it is a dielectric material and does not exhibit plasmon resonances. It is deposited by atomic layer deposition and forms an amorphous lm (alumina). The Al 2 O3 substrate is commercially available sapphire glass. A summary of all material systems is shown in gure 1. In all further gures the pictographs introduced in this gure are used to indicate the corresponding material system. We have chosen the materials for two reasons: IC1-200 can be easily applied via spin-coating and is thus a cheap and straightforward way to coat even complex nanostructures. Atomic layer deposition of alumina is known to produce dense and crack-free lms of well controllable thickness. The material itself is expected to be extremely stable while the defect-free layers also allow for ultimately thin, yet closed lms. For such thin lms of 4 nm thickness the plasmonic near-eld of the structure still signicantly extends into the volume above the cover layer and thus allows for interaction with it for, e.g., sensing applications.

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m 320n

140-

Au 40nm Cr 2nm Suprasil

60nm

bare gold 150nm IC1-200

4nm Al2O3

40nm Al2O3

ambient atmosphere

high laser intensity up to 10 GW/cm2

high temperature up to 1100°C

Figure 1: Geometrical parameters used for the rod nanostructures and the applied covers. In the bottom, visualization of degradation sources, which we applied to determine the damage threshold of our structures. The alumina-coated gold structures withstand heating up to 900 ◦ C in ambient atmosphere, where bare gold rods or TiN rods are already destroyed. Furthermore, the alumina-covered gold rods show a stable optical third harmonic generation at 10 GW/cm2 laser intensity, where bare gold rods already show strong degradation of the third harmonic signal. For the fabrication of the gold rod arrays, standard electron beam lithography is used. The metals are deposited on the suprasil and sapphire substrates by thermal evaporation. The thicknesses are 2 nm chromium as adhesion layer and 40 nm gold on top (see gure 1). The chromium adhesion layer has a major eect on the stability of the gold structures 39 leading to an improved adhesion to the substrate and therefore increased dimensional stability. The width of the rods is chosen to be 60 nm. Their length is varied from 140 nm to 320 nm. All optical measurements are performed with light polarized parallel to the long rod axis. The lattice constant is 500 nm for both directions. The total area of one array is 80 µm x 80 µm. Scanning electron micrographs are shown in gure 2. More SEM images are available in the supporting information. All structures are cleaned after the chemical lift-o by 20 s of oxygen plasma treatment. The IC1-200 coating is deposited by spin-coating (4000 rpm for 40 s) and afterwards annealed at 200 ◦ C for 1 min. Finally, the lm thickness was determined to be 150 nm for all these samples by a stylus proler. For the atomic layer deposition of Al2 O3 (Cambridge NanoTech Savannah 100) trimethylaluminium (TMA) and water are used as precursor gases. The sample is heated to 250 ◦ C during the process, resulting in a growth 5


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rate around 0.1 nm per cycle. TiN nanostructures show strong oxidization at 800 ◦ C, even in an argon-ushed environment. 22 Therefore, as a rst test for our proposed material systems, all covered and uncovered structures are exposed to 800 ◦ C in

ambient atmosphere to investigate the performance in

comparison to TiN. The transmission spectra are measured with a commercial FTIR setup after preparation and after 1, 4, and 8 hours exposed to 800 ◦ C in air. The spectral data from a silicon detector and an InGaAs detector are stitched together, making it possible to measure the wavelength range from 600 nm to 1600 nm. First, the spectral evolution for the dierent material systems are compared for structures with a resonance around 1000 nm. Due to the dierent refractive indices of the surrounding, the length thus varies for dierent covers. The size dependence is discussed later and is depicted in gure 5. 20°C

1h at 800°C

4h at 800°C

8h at 800°C

0.8 0.4

200 nm

0 0.8

1-Transmittance

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0.4 0 0.8 0.4 0 0.8 0.4 0 700

900

1100

before heat treatment

Wavelength (nm)

after 8h at 800°C

Figure 2: Left: Time dependent change of the optical spectra for gold rods with dierent cover layers (from top: bare, 4 nm alumina, 40 nm alumina, 150 nm IC1-200), heated at 800 ◦ C. Right: SEM images before and after heating; the scale bar is always 200 nm. The optical transmission spectra for the bare gold rods are shown in the upper left panel of gure 2. The upper right panels contain SEM micrographs of the rods before exposure to heat and after 8 hours at 800 ◦ C. The initially 220 nm long rods have strongly deformed to 6


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a circular shape, in agreement with literature. 6 The deformation leads to a shrinking along the optically excited direction. This is reected in the more than 300 nm shift of the center wavelength to lower wavelengths. Furthermore, the resonance is rather broad for gold at the respective wavelength. This can be attributed to the very inhomogeneous sizes of the deformed particles. Further SEM micrographs with a wider angle of view are provided in the supporting information. In comparison, the rods coated with 4 nm alumina are still intact. The spectra show only a minor shift of the center wavelength towards shorter wavelengths and a small decrease in the full width at half maximum (FWHM) of the resonance. The third panel displays the 40 nm thick alumina-coated rods. The coating also protects the nanostructure from deformation. A small shift occurs, but towards longer wavelengths. A red shift could occur by an elongation of the nanostructures, however this is unlikely. Alternatively the eective refractive index of the surrounding could change. This may occur due to a kind of sintering eect of the alumina. Another possibility is the formation of a thin gold-alumina intergranular lm. 40 However, as the eect is not visible in case of a thin alumina coating, the reason remains somewhat elusive at this point. It should be stressed, though, that the eect is minor and does by no means inuence our interpretation. Another dierence to the thin alumina coating the dierent measurements are barely discriminable, indicating that a steady state for the particle shape is reached faster. Finally, the IC1-200-coated nanostructures show a slight shift of the resonance to shorter wavelengths. However, a small spectral shift is visible between all measurements. This behavior indicates that the IC1-200 system undergoes small changes for every time step and does not reach a steady state. Besides the inuence of the dierent covers, the inuence of the chromium adhesion layer has to be emphasized. It is known to improve the thermal and dimensional stability of gold nanostructures. It has been shown that without chromium adhesion layer gold starts to already deform at temperatures of 100 ◦ C. 6 Thus the chromium will in all cases contribute to

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the improved stability, in particular for the bare gold case. It has also been reported 39 that the chromium diuses into the gold at elevated temperatures which is expected to partially degrade the plasmonic resonance. Our experiments, however, show a clear improvement of the plasmonic resonance with temperatures right up to the structural break-down of the rod structures. It is thus very dicult to access the inuence of chromium diusion. All three coatings signicantly stabilize the gold resonance and keep the structures dimensionally stable. The alumina coating appears to be the most stable case. To check whether an all-alumina matrix would be even superior, the measurements are also conducted with a sapphire substrate. Intuitively, one would expect the geometry to be even superior with respect to its thermal stability. However, even though this material system prohibits the deformation, there is no major improvement compared to the suprasil substrate. Applicationwise it is even counterproductive, as the strength of the initial resonance is substantially weaker. This is due to the higher refractive index of the substrate ( nsuprasil (1000 nm) = 1.45;

nsapphire (1000 nm) = 1.75). Therefore, the rod length has to be smaller for the antenna to possess the same center wavelength. The smaller gold volume reduces the dipole strength and leads to a weaker resonance. The measurement is shown in the supporting information. To understand the thermal behavior of the dierent material systems, new samples of all four systems are fabricated and are heated at increasing temperatures starting from 100 ◦ C up to 1100 ◦ C. After each step the transmission spectra are recorded. The samples are heated with two dierent heat sources. For temperature from 100 ◦ C to 500 ◦ C the samples are heated on a hot plate for 30 min in 50 ◦ C steps. At higher temperatures ranging from 400 ◦ C to 1100 ◦ C, the samples are heated in a furnace for 60 min in 100 ◦ C steps. The overlapping temperature range was measured to verify that the hot plate temperature was also reached at the position of the nanostructure. The spectra of the complete temperature cycle are displayed in gure 3 a. For the sake of clarity spectra are shown in 100 ◦ C steps. In gure 3 b the center wavelength and the FWHM are extracted from the spectra. To do this for a strongly changing resonance in a rather robust way, the wavelength where the minimal

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transmission occurs is dened as center wavelength. SEM images of the structures after the full temperature cycle are shown in gure 4.

1-Transmittance

a 0.8 0.4 0 0.8 0.4 0 0.8 0.4 0 0.8 0.4 0

700

900

1100

Wavelength (nm)

Center wavelength (nm)

b 1050

FWHM (nm)

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225

850 650

150 75 0

300

600

900

Temperature (°C)

Figure 3: (a) Spectra for gold rods with dierent covers and without cover layer. The rods are subsequently heated at the indicated temperature for 30 min at 100 ◦ C up to 500 ◦ C on a hotplate and 60 min for the temperatures 600 ◦ C up to 1100 ◦ C in a furnace. (b) Extracted center wavelength and FWHM of the transmission spectra. The top panel of gure 3 a shows the evolution of the transmittance spectrum of the bare gold rods. A uniform blue shift of the center wavelength, a decrease of the FWHM, and an increase of the resonance amplitude are visible up to 500 ◦ C. After this temperature the center wavelength shifts rapidly to the blue; the FWHM increases and the amplitude drops. At around 1000 ◦ C the resonance completely vanishes. For every temperature step the spectra change, indicating a steady deformation of the nanostructure for each temperature

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step. In the second panel the spectra of the 4 nm alumina covered gold rods are depicted. The center wavelength of the structure remains unchanged until 700 ◦ C; only the FWHM decreases and the amplitude increases continuously. Even beyond 700 ◦ C, the resonance shifts slightly up to 800 ◦ C. What is more, it remains unchanged at the next temperature step to 900 ◦ C. At 1000 ◦ C the spectrum changes signicantly and vanishes at 1100 ◦ C. The spectrum of the thick alumina-coated structure behaves qualitatively very similar to the thin cover up to 700 ◦ C; the only dierence is a small red shift of the center wavelength. This red shift is also visible for the sample heated constantly at 800 ◦ C (see gure 2). Above 700 ◦ C the spectrum is continuously shifting to shorter wavelengths and starts to broaden above 900 ◦ C. Even after 1100 ◦ C a broad plasmon resonance is observed. For the IC1-200-covered sample, a strong change in the spectra sets in at 700 ◦ C. For temperatures from 350 ◦ C to 700 ◦ C, the center wavelength remains constant. At 350 ◦ C a small shift of the center wavelength is visible. This may be due to a shift of the refractive index of the IC1-200. The hard bake temperature of IC1-200 is 400 ◦ C. However, it was only hard baked at 200 ◦ C, which is sucient for normal electron lithography usage. To dene a thermal damage threshold based on the optical measurement is ambiguous. The easiest measure would be the center wavelength, which is directly related to the length of the resonantly excited rod. 41 Nevertheless, in gure 2 it is shown that despite a change in the center wavelength the shape of the structure remained nearly the same over hours. Also the FWHM is no ideal measure. For a single rod it would resemble the lifetime of the plasmon. The investigated structures are arrays of around 25000 individual rods. Therefore, the FWHM reects averaging eects as well. All four systems show a uniform decline of the FWHM in the beginning, which may be due to a sintering process. Sintering of metals can lead to a reduced resistance and a reduction of grain boundaries, 36,42 which may result in a reduced FWHM. Beyond a certain temperature an increase in the FWHM is visible. This is most likely due to an increased length distribution of the rods, caused by the individual

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deformation process. For the bare gold and the IC1-200-coated sample the shift in center wavelength and the increase of the FWHM occur simultaneously, for the bare gold rods at 500 ◦ C and the IC1-200-coated rods at 700 ◦ C. However, the alumina-coated samples show a decreasing or constant FWHM up to 900 ◦ C and 1000 ◦ C, while their center wavelength is already shifted signicantly. The shift may also be partly due to a change of the eective refractive index of the environment. This would imply that the cover layer changed, since changes in the bulk substrate are more unlikely. In combination with the long term measurements at 800 ◦ C the alumina-coated structures are long term stable after a short annealing time. Therefore, we estimate the damage threshold to be in the range from 800 ◦ C to 1000 ◦ C for these structures. There are some distinct dierences visible between the previous measurement at constant temperature of 800 ◦ C (see gure 2) and the measurement at 800 ◦ C of this temperature series. The bare gold system and the thin alumina system look comparable. However, the thick alumina as well as the IC1-200 sample show stronger degradation in the temperature series than in the measurement at constant temperature. This may be caused by the different thermal expansion properties of the dierent materials, which can lead to damages in the protective layer in every heating and cooling cycle. Furthermore, despite the similar fabrication conditions, the thermal stability will strongly depend on the interface properties between the coating, the gold structures, and the substrate. The adhesion of spin-coatable dielectrics to sample surfaces is known to critically depend on the exact surface conditions. Similarly, the formation of molecular layers during the ALD process is strongly dependent on the surface chemistry. While we tried to keep these properties as similar as possible by applying the plasma cleaning treatment, small dierences are expected to already signicantly inuence the "sealing quality" of the overcoated and overgrown layers. The coatings can stabilize the gold structures up to a temperature of at least 700 ◦ C. This nding is supported by a report on carbon-coated gold nano rods, where structural deformations are visualized by transmission electron micrography starting at around 700 ◦ C. 33

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The very similar temperature range for very dierent coatings suggest an eect independent of the coating material. For temperatures higher than 700 ◦ C, degradation apparently starts to set in even for the covered structures. Especially the alumina coating prohibits a strong deformation up to around 900 ◦ C and the two thick coatings allow for the observation of a plasmon resonance up to a temperature of 1100 ◦ C, which is above the melting point of bulk gold at 1064 ◦ C. After the complete temperature cycle up to 1100 ◦ C the samples are imaged with SEM. The micrographs are depicted in gure 4. The upper four micrographs show a top-down view of the dierent samples. The bare gold and the thin alumina-coated samples show a complete destruction of the rods. Also 100 µm size marker structures completely vanished. Only strongly deformed structures can be observed; we suspect them to be parts of the chromium adhesion layer. We assume that the gold liqueed and completely diused from the nanostructured area and accumulated in the surrounding, a process known as Oswald ripening. 43 However, in the close proximity of the nanostructured area, no accumulations of the gold are found. This nding is in accordance with the vanishing of the plasmon resonance in the transmittance spectrum.

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a

E

500nm

b

E

Figure 4: (a) SEM top down view after samples were heated up to 1100 ◦ C. The measured polarization is indicated as red arrow. (b) Cross-sectional SEM side view; the cross-section has been milled by focused ion beam; the scale bar is always 500 nm. The voids are visible as black features below the gold dots. The samples with the two thicker cover layers show a remaining plasmon resonance even after the complete temperature cycle. However, the resonance is extremely broad and quite far blue shifted. Thus, based on the optical measurements it is expected that the gold structures have shrunk and their size distribution is quite inhomogeneous. For both samples the rods can be clearly observed in the top-down SEM image and have changed their shape as expected. To obtain a side view of the structure, we applied focused ion beam (FIB) milling (Raith ionLINE Plus). The cover layer, as well as part of the structures, have been milled o in order to allow access to the side facet of the buried structures. Afterwards, the sample is investigated by SEM (gure 4 b). For the alumina coating the cover layer is still intact, yet the gold particles have strongly deformed to a spherical shape inside the

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cover layer. Interestingly, it appears as if the alumina cover retained its shape and still shows the original extrusions of the rod like gold structure at the air interface. The gold particles seem to have detached from the substrate surface, leaving a dark void below them. However, it is to mention, that the inuence of the FIB cut on the exact particle position is unclear. Furthermore, is is ambiguous where the exact alumina-suprasil interface is located and therefore how strongly the particles have detached. It also appears for the IC1-200 cover, that the structure has changed. The original cover lm thickness was 150 nm. The lm either has shrunk, or the particles have moved inside the cover layer, away from the substrate. The gold particles appear to have formed round particles, which would imply that they partly detached from the substrate. Further SEM images can be found in the supporting information. To investigate the size dependence of the dimensional stability, the experiment that observes the temperature evolution at constant temperature of 800 ◦ C (gure 2 a), as well as the increasing temperature series are carried out with rods of varying length (gure 5 b). The rod length is varied in 20 nm steps from 140 nm to 320 nm. Only three of the lengths are presented here, because there is negligible dierence between the dierent sizes. All sizes are shown in the supporting information. In gure 5 a the extracted center wavelength and FWHM of the transmittance spectra for uncovered rods and rods covered with 4 nm alumina are depicted. The structures are exposed to constant 800 ◦ C. The bare gold rods exhibit a strong change of resonance wavelength as well as FWHM in the rst time step. In the subsequent heating steps, no signicant change is visible, in accordance with the previous results.

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a

Center wavelength (nm)

Bare gold 1300

FWHM (nm)

4nm Al2O3

140nm 240nm 320nm

1000 700 450 350 150 0

180

360

0

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360

Time at 800°C (min) Center wavelength (nm)

b

FWHM (nm)

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Bare gold 1300 1000

4nm Al2O3

140nm 240nm 320nm

700 450 350 150 0

300 600 900

0

300 600 900

Temperature (°C)

Figure 5: Change of the center wavelength and FWHM of the transmittance spectra of rods width dierent length. The left column depict bare gold rods and the right column shows rods with a 4 nm alumina cover exposed to 800 ◦ C over a time of 480 min (a) and increasing temperatures (b). The samples in (b) are kept at the indicated temperature for 30 min for temperatures from 100 ◦ C to 500 ◦ C and 60 min for higher temperatures. Furthermore, the aspect ratio from length to width plays an essential role for the strength of the change. The smaller the aspect ratio (the shorter the particle), the less the particle will shrink during the deformation to a circular shape. For the alumina-coated structures, a decrease in the center wavelength as well as in the FWHM is visible. Nevertheless the change is far less pronounced and the inuence of the aspect ratio on the strength of change is visible. The size dependence of the temperature cycle presented in gure 3 is depicted in gure 5 b. Also here all the previously observed features are visible. The aspect ratio determines the strength of the change, but not the temperature stability. Based on these

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observations, a major size-dependent eect concerning the thermal stability of the gold rods can be excluded. The data for the two thick covers are in excellent agreement with this conclusion and can be found in the supporting information. a Fibre Broadened Yb:KGW

Spectrometer Sample

44MHz, 180mW, 16fs

4nm Sapphire

bare

KG Filter

4nm Sapphire

bare

1.1

0.8

0.9

0.5

0.7

1-Transmittance

c

b TH Intensity normalized

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0.2 0

10

20

Time (min)

30 950

1050

1150

Wavelength (nm)

Figure 6: (a) Experimental setup for the third harmonic spectroscopy; (b) TH intensity of covered and bare gold rods exposed to 10 GW/cm 2 laser radiation (180 mW, beam FWHM 32 µm). (c) Transmittance spectra of the nanostructure types before (solid line) and after 30 min laser exposure (dashed line); the laser spectrum is displayed in gray. Intense laser illumination at high repetition rates and low pulse energies generates heat and strong local electric elds in plasmonic nanostructures, causing signicant damage to the investigated nanostructures. Having our ndings in mind, we expect that the increased stability of our covered nanostructures should also lead to an increased damage threshold for laser illumination. To test this assumption, the material systems are exposed to 10 GW/cm 2 of laser radiation, well above the uence normally used for nonlinear spectroscopy. 44 As a measure of the stability, the temporal evolution of the generated third harmonic (TH) intensity is studied. Apart from the fact that many applications of nonlinear plasmonics in fact use TH generation, we also choose this measure as the signal is known to crucially depend on the exact shape of the plasmonic resonance. 45 The measurement is performed with an in-house build laser source (gure 6 a). 21,46 The laser source provides a spectrum 16


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ranging from 950 nm to 1150 nm with a pulse duration of 16 fs, a repetition rate of 44 MHz, and a maximum average power of 180 mW before the focusing lens in front of the sample. To remove all spectral components below 700 nm, a 3 mm thick Schott RG715 lter is placed before the sample. The beam at the focal point has a FWHM of 32 µm, measured by a knife edge test. The TH is detected with a Peltier-cooled CCD-Detector, after the fundamental beam is strongly attenuated (Schott glasses, 6 mm KG5 and 3 mm KG4 ). The measured TH intensity is shown in gure 6 b. The TH response is normalized for all structures to the TH of the respective structure at time zero. For both alumina-coated rods a substantial increase of the TH in the rst time step is visible. Afterwards the TH intensity remains constant for the rest of the measurement. The linear spectrum should reect this change. For both alumina-coated samples, a rather small decrease of the FWHM is visible. However, in the anharmonic oscillator model for nonlinear plasmonic processes the FWHM enters at the sixth power, 3 indicating that already small changes in the FWHM, i.e., the quality factor of the resonance, lead to signicant changes in the signal strength. Similar results have been reported recently. 21 The TH of the bare gold structure and the IC1-200-covered structure continuously decrease. The reason is the change of the linear spectra, which changes and shifts to shorter wavelengths out of the overlap with the laser spectrum, resulting in a reduced nonlinear signal. We have shown that the alumina overcoated structures exhibit a signicantly increased damage threshold compared to the bare structures, but also when compared to the IC1-200coated structures. The changes of the signal strength (i.e, the increased TH) is related to the change in the quality factor which most likely stems from a local heating of the sample, very similar to the increase in quality factor observed in the heating experiments discussed above. In summary, we presented three dierent gold based plasmonic systems, which combine the thermal stability of a refractory material and the chemical stability of gold. The alumina coating nearly doubles the thermal damage threshold compared to the bare gold and

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outperforms titanium nitride systems. The exact thermal damage threshold varies slightly from sample to sample, yet it is signicantly enhanced from 500 ◦ C to around 900 ◦ C. What is more, we demonstrate the superior damage threshold for intense laser illumination at least up to 10 GW/cm2 . The thickness of the protection layer can be as thin as 4 nm, without losing its protective properties. This thickness is thin enough to still access the enhanced near eld of the plasmonic structure. Therefore, the cover does not hamper the usage of the system in refractive index sensing applications. All in all, our concept extends the scope for gold nanostructures to high temperature, ambient atmosphere, and intense illumination applications, overcoming the chemical stability limitations of refractory materials and the thermal stability limitations of bare gold.

Acknowledgement We acknowledge support from ERC (Complexplas), BMBF, DFG (SPP 1839), and BW Stiftung. We thank Marion Hagel for fabricating the ALD layers. Also we would like to thank Shahin Bagheri and Rotraut Merkle for fruitful discussions about refractory materials.

Supporting Information Available Transmission spectra of an all-alumina system; wide angle SEM images of the dierent material systems after 8 h at 800 ◦ C and after the full temperature cycle up to 1100 ◦ C; temperature dependence of the resonance for varying rod lengths. This material is available free of charge via the Internet at http://pubs.acs.org/ .

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References (1) Catchpole, K.; Polman, A. (2) Schultz, D. A.

Optics Express

2008,

Current Opinion in Biotechnology

16, 2179321800. 2003,

(3) Hentschel, M.; Utikal, T.; Giessen, H.; Lippitz, M.

14, 1322.

Nano Letters

2012,

(4) Yin, Y.; Li, Z.-Y.; Zhong, Z.; Gates, B.; Xia, Y.; Venkateswaran, S.

Chemistry

2002,

12, 37783782.

Journal of Materials

12, 522527.

(5) Thyagarajan, K.; Santschi, C.; Langlet, P.; Martin, O. J.

Advanced Optical Materials

2016, 4, 871876.

(6) Petrova, H.; Juste, J. P.; Pastoriza-Santos, I.; Hartland, G. V.; Liz-Marzán, L. M.; Mulvaney, P.

Physical Chemistry Chemical Physics

(7) Liu, Y.; Mills, E. N.; Composto, R. J.

2006, 8, 814821.

Journal of Materials Chemistry

2009,

19, 2704

2709. (8) Mohamed, M. B.; Ismail, K. Z.; Link, S.; El-Sayed, M. A.

Chemistry B

1998,

102, 93709374.

(9) Gou, L.; Murphy, C. J.

Chemistry of Materials

(10) Wang, Y.; Teitel, S.; Dellago, C.

Nano Letters

2005,

2000,

17, 36683672.

2005, 5, 21742178.

(11) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A.

B

The Journal of Physical

The Journal of Physical Chemistry

104, 61526163. Nano Letters

2013,

The Journal of Physical Chemistry B

2005,

(12) Kuhlicke, A.; Schietinger, S.; Matyssek, C.; Busch, K.; Benson, O.

13, 20412046. (13) Inasawa, S.; Sugiyama, M.; Yamaguchi, Y.

109, 31043111. 19


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

(14) Gentile, M.; Hentschel, M.; Taubert, R.; Guo, H.; Giessen, H.; Fiebig, M.

Physics B: Lasers and Optics

Applied

105, 149162.

2011,

Optical Materials Express

(15) Naik, G. V.; Kim, J.; Boltasseva, A.

2011, 1, 10901099.

Advanced Materials

(16) Naik, G. V.; Shalaev, V. M.; Boltasseva, A.

(17) Park, J. H.; Han, S. E.; Nagpal, P.; Norris, D. J.

ACS Photonics

2013,

25, 32643294.

2016, 3, 494500.

(18) Arpin, K. A.; Losego, M. D.; Cloud, A. N.; Ning, H.; Mallek, J.; Sergeant, N. P.; Zhu, L.; Yu, Z.; Kalanyan, B.; Parsons, G. N.; Girolami, G. S.; Abelson, J. R.; Fan, S.; Braun, P. V.

Nature Communications

2013, 4, 2630.

(19) Hanham, S.; Fernández-Domínguez, A.; Teng, J. H.; Ang, S.; Lim, K.; Yoon, S. F.; Ngo, C.; Klein, N.; Pendry, J.; Maier, S. A. (20) Guler, U.; Boltasseva, A.; Shalaev, V. M.

Advanced Materials

Science

2014,

2012,

24 .

344, 263264.

(21) Gui, L.; Bagheri, S.; Strohfeldt, N.; Hentschel, M.; Zgrabik, C. M.; Metzger, B.; Linnenbank, H.; Hu, E. L.; Giessen, H.

Nano Letters

2016,

16, 57085713.

(22) Bagheri, S.; Zgrabik, C. M.; Gissibl, T.; Tittl, A.; Sterl, F.; Walter, R.; De Zuani, S.; Berrier, A.; Stauden, T.; Richter, G.; Hu, E. L.; Giessen, H.


2015, 5, 26252633.

(23) Wang, Y.; Capretti, A.; Dal Negro, L.


2015, 5, 24152430.

(24) Briggs, J. A.; Naik, G. V.; Petach, T. A.; Baum, B. K.; Goldhaber-Gordon, D.; Dionne, J. A.

Applied Physics Letters

2016,

108, 051110.

(25) Li, W.; Guler, U.; Kinsey, N.; Naik, G. V.; Boltasseva, A.; Guan, J.; Shalaev, V. M.; Kildishev, A. V.

Advanced Materials

2014,

26, 79217921.

(26) Guler, U.; Ndukaife, J. C.; Naik, G. V.; Nnanna, A. A.; Kildishev, A. V.; Shalaev, V. M.; Boltasseva, A.

Nano Letters

2013,

13, 60786083. 20


Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(27) Naik, G. V.; Schroeder, J. L.; Ni, X.; Kildishev, A. V.; Sands, T. D.; Boltasseva, A.

Optical Materials Express (28) Zgrabik, C. M.; Hu, E. L.

2012, 2, 478489.


(29) Molesky, S.; Dewalt, C. J.; Jacob, Z.

Optics Express

(30) Mitsuo, A.; Uchida, S.; Nihira, N.; Iwaki, M.

2015, 5, 27862797. 2013,

21, A96A110.

Surface and Coatings Technology

1998,

103, 98103. (31) Liu, J.; Guler, U.; Lagutchev, A.; Kildishev, A.; Malis, O.; Boltasseva, A.; Shalaev, V. M.


2015, 5, 27212728.

(32) Albrecht, G.; Hentschel, M.; Kaiser, S.; Giessen, H.

ACS Omega

(33) Khalavka, Y.; Ohm, C.; Sun, L.; Banhart, F.; Sönnichsen, C.

Chemistry C

2007,

2017, 2, 25772582.

The Journal of Physical

111, 1288612889.

(34) Kujala, S.; Caneld, B. K.; Kauranen, M.; Svirko, Y.; Turunen, J.

Optics Express 2008,

16, 1719617208. (35) Chen, Y.-S.; Frey, W.; Kim, S.; Homan, K.; Kruizinga, P.; Sokolov, K.; Emelianov, S.

Optics Express

2010,

18, 88678878.

(36) Bosman, M.; Zhang, L.; Duan, H.; Tan, S. F.; Nijhuis, C. A.; Qiu, C.-W.; Yang, J. K.

Scientic Reports

2014, 4, 5537.

(37) Sivis, M.; Duwe, M.; Abel, B.; Ropers, C.

Nature Physics

(38) Mesch, M.; Metzger, B.; Hentschel, M.; Giessen, H.

Nano Letters

(39) Pan, J.; Pafchek, R. M.; Judd, F. F.; Baxter, J. B.

packaging

2006,

2013, 9, 304309. 2016,

IEEE transactions on advanced

29, 707713.

(40) Baram, M.; Chatain, D.; Kaplan, W. D.

Science

21

16, 31553159.

2011,


332, 206209.

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

(41) Ringe, E.; Langille, M. R.; Sohn, K.; Zhang, J.; Huang, J.; Mirkin, C. A.; Van Duyne, R. P.; Marks, L. D.

The Journal of Physical Chemistry Letters

2012,

3, 14791483. (42) Allen, M. L.; Aronniemi, M.; Mattila, T.; Alastalo, A.; Ojanperä, K.; Suhonen, M.; Seppä, H.

Nanotechnology

2008,

19, 175201.

(43) Sundar, A.; Hughes, R.; Farzinpour, P.; Gilroy, K.; Devenyi, G.; Preston, J.; Neretina, S.

Applied Physics Letters

2012,

100, 013111.

(44) Metzger, B.; Hentschel, M.; Lippitz, M.; Giessen, H.

Optics Letters

2012,

37, 4741

4743. (45) Lippitz, M.; van Dijk, M. A.; Orrit, M.

Nano Letters

(46) Metzger, B.; Steinmann, A.; Giessen, H.

2005, 5, 799802.

Optics Express

22


2011,

19, 2435424360.

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Graphical TOC Entry no cover Au 4nm Al2O3 Au 20°C

800°C Temperature

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Refractory Plasmonics without Refractory Materials - Nano Letters

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